This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-392891, filed Dec. 25, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a color cathode ray tube provided with a shadow mask.
2. Description of the Related Art
In general, a color cathode ray tube is constructed so that electron beams emitted from an electron gun are deflected in the horizontal and vertical directions by means of horizontally and vertically deflecting magnetic fields that are generated by a deflection yoke, and scan a phosphor screen horizontally and vertically through a shadow mask, thereby displaying a color image.
The shadow mask is used for color sorting such that the electron beams are landed on specific phosphor layers, and is located in a predetermined position relative to the phosphor screen. However, the shadow mask undergoes thermal expansion as it is hit by the electron beams, so that its position relative to the phosphor screen is shifted. Examples of measures to counter this problem are proposed in Jpn. Pat. Appln. KOKAI Publications Nos. 60-243945 and 5-41179 and Jpn. UM Appln. KOKAI Publication No. 2-143759. According to these measures, a part of the shadow mask that is susceptible to thermal expansion is doubled in structure so that its heat capacity and strength are increased.
In prevailing color cathode ray tubes, the shadow mask is formed of a material with a low coefficient of thermal expansion, such as an Invar material, or its curved surface is deliberately shaped to cope with the problem of thermal expansion.
On the other hand, flat tubes have recently started to spread. One such flat tube is a cathode ray tube of which the outer surface of the panel is substantially flat, having a radius of curvature of 100 m or more. Usually, an effective portion of the shadow mask in which electron beam passage apertures are formed has a flat shape corresponding to the shape of the inner surface of the panel. Accordingly, the shadow mask of the flat tube has a curvature smaller than that of a shadow mask of a conventional color cathode ray tube of which the outer surface of the panel is curved.
If the curvature of the shadow mask is reduced in this manner, it is hard for the shadow mask itself to maintain its curved surface, resisting the shadow mask own weight or external force. If the curved surface retention of the shadow mask (hereinafter referred to as mask strength) is low, the curved surface of the shadow mask is inevitably deformed by a small external force that acts on it during the manufacture or transportation. A deformation of the shadow mask changes the distance between the electron beam passage apertures and the inner surface of the panel. In consequence, the electron beams fail to be landed on the specific phosphor layers, thereby causing a color drift.
If the mask strength is low, moreover, the curved surface of the shadow mask easily resonates with vibrations, such as the sound from a speaker, when the mask is incorporated in a TV set or the like. If the shadow mask resonates, unnecessary light and shade appear on the screen plane, leading to a lower quality image.
The easiest method to prevent lowering of the mask strength is to thicken the shadow mask. However, if the thickness of the shadow mask is increased, etching control during the manufacture of the mask is so difficult that the electron beam passage apertures are subject to substantial irregularity in diameter. In consequence, the yield of production of shadow masks and color cathode ray tubes lowers, uneven display is caused, and the image quality level is lowered.
The present invention has been contrived in consideration of these circumstances, and its object is to provide a color cathode ray tube provided with a shadow mask having satisfactory strength and capable of producing images of higher quality levels.
In order to achieve the above object, a color cathode ray tube according to an aspect of the invention comprises a panel provided with a phosphor screen, an electron gun configured to emit an electron beam toward the phosphor screen, and a shadow mask assembly located between the phosphor screen and the electron gun. The shadow mask assembly includes a shadow mask body having a rectangular effective portion opposed to the phosphor screen and formed having a number of electron beam passage apertures, the effective portion having a major axis and a minor axis passing through the center thereof and extending at right angles to each other, a mask frame to which the periphery of the shadow mask body is fixed, and an auxiliary mask in the form of a strip long in the direction of the minor axis, fixed to a region containing the minor axis of the effective portion, and having a number of electron beam passage apertures communicating individually with the electron beam passage apertures of the effective portion.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.